High voltage circuit breakers are used in the transmission and distribution of electrical energy. These circuit breakers are located inside of tanks which contain SF6 gas. The SF6 gas acts as an electrically insulating gas to prevent arcing. The purpose of the gas is to control arcing across electrical contacts during opening of closing of contacts in the breaker.
SF6 gas monitors are known in the art as shown in U.S. Pat. No. 4,489,592 Pacanowski et al., U.S. Pat. No. 4,802,370 EerNisse et al., U.S. Pat. No. 5,388,451 Stendin et al., U.S. Pat. No. 6,263,914 Meyer et al., and U.S. Pat. No. 6,651,483 Meyer et al. In the above referenced patents, SF6 density in a container which houses electrical switch gear is monitored by observing density and comparing the measurement to an industry standard. Monitoring of density change necessarily involves computations of the quantity of gas present in the container. The quantity of gas present in the container is determined by measuring gas pressure and gas temperature and applying the perfect gas equation.PV=nRT
In the prior art, the temperature inside of the electrical apparatus container has been measured from the outside. Measurement from the outside is necessary when a temperature measuring transducer is not incorporated in the container at the time of manufacture. Because of arcing considerations, and conditions inside the container, it is generally impractical to measure temperature of the gas directly inside of the container, especially in the case where it is desired to provide SF6 monitoring on an existing tank which does not include a provision for temperature measurement inside the tank. For this reason, the prior art measures temperature of the tank skin, and attempts to estimate the true temperature inside the tank in order to as closely as possible estimate the true temperature of the gas inside the tank.
In the prior art devices, measurements are made only at specific times, and information relating to density loss as a function of time or trends of density loss are not made.
Daily variations of SF6 pressure due to temperature change, preclude using pressure measurements alone in detecting smaller leaks. The ideal gas law is used to adjust for pressure measurements alone. Under the ideal gas law, the ideal pressure temperature relationship of SF6 is linear.
In electric utilities, several hundred thousand pounds of SF6 per year are lost due to faulty seals. The loss of this expensive dielectric gas is an environmental concern and is subject to routine pressure inspections to determine leakage. Since SF6 is a potent greenhouse gas, early detection of leaks can prevent environmental degradation, reduce replacement costs and allow maintenance staff to implement repair before an emergency condition develops. Slow leaks from mechanical seals are difficult to detect and current technology in the electrical industries lack the requisite sensitivity.